CID Electromagnetic Compatibility & EMI Control 2 — Questions and Answers
Question 1: In CISPR 32 and FCC Part 15 emissions testing, what is the key difference between Class A and Class B limits?
- Class A limits are stricter and apply to residential equipment; Class B applies to industrial
- Class B limits are stricter and apply to residential equipment; Class A applies to commercial/industrial (Correct answer)
- Both classes have identical limits but differ in measurement distance
- Class A covers conducted emissions only; Class B covers radiated emissions only
Correct answer: Class B limits are stricter and apply to residential equipment; Class A applies to commercial/industrial
Class B limits are more stringent and apply to equipment intended for use in residential environments, while Class A limits apply to commercial and industrial settings where more interference may be tolerable.
Question 2: A common-mode choke in a cable assembly primarily attenuates:
- Differential-mode signals intended to travel along the cable
- Common-mode currents flowing in the same direction on both conductors (Correct answer)
- The DC resistance of the cable
- The characteristic impedance of the transmission line
Correct answer: Common-mode currents flowing in the same direction on both conductors
A common-mode choke presents high impedance to currents flowing in the same direction (common-mode) while passing differential signals largely unimpaired, suppressing conducted EMI.
Question 3: What is the primary EMC advantage of a multilayer PCB with dedicated ground and power planes over a two-layer board?
- Lower manufacturing cost and simpler assembly
- Reduced loop inductance of the power distribution and signal return paths (Correct answer)
- Easier visual inspection of all traces
- Higher trace current-carrying capacity due to more copper layers
Correct answer: Reduced loop inductance of the power distribution and signal return paths
Dedicated reference planes in multilayer boards create a very low inductance power distribution network and minimise the area of signal return current loops, dramatically reducing both radiated and conducted EMI.
Question 4: Which cable routing technique most reduces radiated emissions from an interconnect cable acting as an unintentional antenna?
- Routing cables in free air away from the chassis
- Bundling cables together and routing them tightly along a grounded chassis (Correct answer)
- Increasing the length of the cable to reduce its resonant frequency
- Using high-gauge wire to increase the cable's DC resistance
Correct answer: Bundling cables together and routing them tightly along a grounded chassis
Routing cables along a grounded chassis provides a nearby return path that reduces loop area and shields the cable from radiating, significantly lowering EMI.
Question 5: When performing conducted emissions measurements per CISPR standards, a LISN (Line Impedance Stabilization Network) is used to:
- Filter all noise from the AC power line before it reaches the EUT
- Provide a defined, repeatable impedance at the AC input of the EUT and couple EMI to the measurement receiver (Correct answer)
- Supply regulated DC power to the equipment under test
- Simulate the impedance of a transmission line on the PCB
Correct answer: Provide a defined, repeatable impedance at the AC input of the EUT and couple EMI to the measurement receiver
A LISN standardizes the impedance seen at the power port of the equipment under test and provides a safe, calibrated coupling point for the spectrum analyzer or receiver to measure conducted emissions.
Question 6: A ferrite bead placed in series on a power supply line in a PCB design primarily provides:
- Low impedance to low-frequency DC power while presenting high impedance to high-frequency noise (Correct answer)
- A low-pass filter for signals below 1 MHz
- A means of increasing the inductance of a differential pair
- A surge protection device for electrostatic discharge events
Correct answer: Low impedance to low-frequency DC power while presenting high impedance to high-frequency noise
Ferrite beads are lossy inductors that pass DC and low-frequency currents with minimal resistance while converting high-frequency noise into heat, making them effective EMI filters on power lines.
Question 7: Which statement correctly describes the relationship between a signal's edge rate (rise/fall time) and its EMI potential?
- Slower edge rates generate more high-frequency harmonics and greater EMI
- Faster edge rates produce higher-frequency harmonic content and generally increase radiated EMI (Correct answer)
- Edge rate has no effect on the frequency spectrum of EMI emissions
- Only the signal amplitude, not the edge rate, determines EMI magnitude
Correct answer: Faster edge rates produce higher-frequency harmonic content and generally increase radiated EMI
Faster signal transitions generate richer high-frequency harmonic content because the spectral energy extends to higher frequencies (approximately 1/πtr), increasing radiated EMI from traces and cables.
CID Electromagnetic Compatibility & EMI Control 2
In CISPR 32 and FCC Part 15 emissions testing, what is the key difference between Class A and Class B limits?